fix cors problems with newsfeed articles (#2940)

solves #2840 as far as possible. There could still be errors on the
embedded iframe when the owner of the site has set `X-Frame-Options` or
`Access-Control-Allow-Origin` headers (as already mentioned in the
docs).

modules/default/newsfeed/newsfeed.js

@@ -42,6 +42,14 @@ Module.register("newsfeed", {
 		dangerouslyDisableAutoEscaping: false
 	},
 
+	getUrlPrefix: function (item) {
+		if (item.useCorsProxy) {
+			return location.protocol + "//" + location.host + "/cors?url=";
+		} else {
+			return "";
+		}
+	},
+
 	// Define required scripts.
 	getScripts: function () {
 		return ["moment.js"];
@@ -142,14 +150,19 @@ Module.register("newsfeed", {
 			sourceTitle: item.sourceTitle,
 			publishDate: moment(new Date(item.pubdate)).fromNow(),
 			title: item.title,
-			url: item.url,
+			url: this.getUrlPrefix(item) + item.url,
 			description: item.description,
 			items: items
 		};
 	},
 
 	getActiveItemURL: function () {
-		return typeof this.newsItems[this.activeItem].url === "string" ? this.newsItems[this.activeItem].url : this.newsItems[this.activeItem].url.href;
+		const item = this.newsItems[this.activeItem];
+		if (item) {
+			return typeof item.url === "string" ? this.getUrlPrefix(item) + item.url : this.getUrlPrefix(item) + item.url.href;
+		} else {
+			return "";
+		}
 	},
 
 	/**